Closure of Remote Historic Underground Mines in Desert Environments

Transcription

1 Closure of Remote Historic Underground Mines in Desert Environments James A. Cremeens Agapito Associates, Inc. Michael H. Rauschkolb U.S. Borax, Inc SME Annual Meeting and Exhibit

2 Geotechnical solutions were developed for closure of two historic underground borate mines in the deserts of California The Old Borate mining district located in the Mojave Desert near Barstow California Mined from 1890 to 1907 The Lila C. Mine near Death Valley California Mined from 1907 to 1916 and 1920 to 1926

3 CALIFORNIA N Location of the Old Borate and Lila C. Mine Sites San Francisco Lila C. Mine Site Death Valley Old Borate Mine Site Los Angeles San Bernardino

4 Big Borate Canyon

5 Big Borate Canyon post-1898

6 Haulage Adit

7 Decline Shaft

8 Dugout Dwelling

9 Underground at Old Borate (Circa 1902 from Reynolds 1999)

10 Prevent public access Closure Criteria Protect wildlife Minimize impact to fragile desert landscape Assure environmental compatibility Optimize use of native materials

11 Wildlife Protection Conducted by bat and desert tortoise specialists Daytime internal examination and night bat surveys with night vision equipment and ultrasonic detectors No tortoise sign found within mines Bats or bat sign found in most of the mine openings Mine openings fenced to allow bat exit but not re-entry

12 Pre-Construction Design ADITS Collapse and Regrade Polyurethane Foam SHAFTS and STOPES Pre-Cast Concrete Panel Caps

13 Pre-cast Concrete Panel Design Survey Monument Armor Finished Slope with Cobbles and Boulders Surficial Material Backfill 1 3 Final Grade (3H:1V) Original Ground Extremely Weathered Siltstone Unweathered Siltstone Excavation (1.5H:1V) Precast Concrete Panels Shaft Opening Backfill with Mine Waste Rock (optional) Panel Abutment Joint 48" Typ. 48" Typ. 12" Typ. Prepare Smooth Foundation Surface and Shim Such That Panel Edges are Flush and Panels are Coplanar Panel Foundation Shaft Opening 2L

14 Field Conditions Strength of weak siltstone and claystone did not increase with depth Low bearing capacity Foundation could be easily eroded Ground caved and grew laterally with excavation Excavated mine openings were larger than surface expression Large spans for pre-cast concrete panels Increased number of concrete panels Would have required steel beam support for large spans

15 Final Design SHAFTS and STOPES Boulder Plug ADITS Collapse and Regrade with Boulder Plug Polyurethane foam

16 Components of a Typical Boulder Plug Closure Survey Monument Surficial Material Backfill Final Grade Original Ground Extremely Weathered Siltstone Unweathered Siltstone Excavation (1H:2V) Boulder Plug

17 Typical Adit Boulder Plug Closure Regraded to Original Slope or Flatter Survey Monument Original Ground Adit Portal Excavation (1.5H:1V) (Collapse) Adit Surficial Material Backfill Boulder Backfill

18 Excavating a Stope

19 Typical Boulder Plug Placement Sequence Placing Initial Boulders Completed Boulder Plug

20 Typical Boulder Plug Placement Sequence Beginning Backfill and Regrade Placing Final Soil Cover

21 Summary of Old Borate Closure 120 mine openings closed in 5-week period Boulder plugs satisfied design criteria Closure costs were significantly reduced by using native materials Lower unit costs = more mine openings closed

22 Lila C. Mine Site Circa 1907 AGAPITO AGAPITO ASSOCIATES, ASSOCIATES, INC. INC. Consulting Engineers

23 Main Haulage Level of the Lila C. Mine Circa 1910

24 Stope at Lila C.

25 Decline shaft at Lila C.

26 Open Stope at Lila C.

27 Wildlife Protection Conducted by bat and desert tortoise specialists No tortoise sign found within mines Surveys revealed four species of bats in the mines Two bat Species of Special Concern found Bat accessible designs were included in closure

28 Bat Accessible Closures Bat gates and cupolas designed by specialty contractor to protect bat access, preserve ventilation, and maintain temperature Underground surveys conducted to optimize bat accessible design

29 Closure Design SHAFTS and STOPES Rock Mattress AAI innovation ADITS Collapse and Regrade with Boulder Plug Polyurethane foam

30 Native Ground Original Grade Native Ground Partially Collapsed and Backfilled Stope Lila C. Mine Typical Profiles Pre-closure Daylighted Stope Configuration Rock Mattress Excavation and Construction of a Daylighted Stope Closure Final Grade Limits of Excavation (1H:1V) Native Ground Rock Mattress Gabions Backfilled Stope Backfill

31 Top Layer Gabions Bottom Layer Gabions Rock Mattress Construction for a Daylighted Slope Existing Open Stope C L Bottom and Top Layer Gabions In Place

32 attress Construction for a Daylighted Stope a) Pre-construction b) Grading Hanging Wall

33 Rock Construction for a Daylighted Stope c) Wheel Roll Compaction d) Gabion Construction

34 Rock Construction for a Daylighted Stope e) Filling Gabions with Native Rock f) Backfilling Over Gabion Rock Mattress

35 Rock Construction for a Daylighted Stope g) Partially Backfilled Rock Mattress h) Final Grade and Armored Slope

36 Rock Construction for a Shaft Closure Plywood Barrier to Keep Debris out of Shaft During Excavation Rock Debris on Plywood Barrier Shaft Perimeter a) Shaft after Surface Preparation for Rock Mattress

37 Rock Construction for a Shaft Closure Shaft Perimeter b) First Layer of Gabions of Shaft

38 Rock Construction for a Shaft Closure Second Layer of Gabions First Layer of Gabions c) First and Second Layers of Rock-filled Gabions Completed Rock Mattress at Shaft

39 Rock Construction for a Shaft Closure d) Load Test on a Rock Mattress at Shaft

40 Rock Construction for a Shaft Closure Backfill over L26a L26b e) Backfill over Rock Mattress

41 Typical Foam Seal Construction a) Preparing Backing Lattice b) Applying Foam c)completed Foam Application without Backfill

42 Bat Gate

43 Construction and Final Configuration of Bat Cupola a) Landing Ring and First Course Concrete Pipe in Place b) Completed/ Backfilled Shaft Extension And Bat Cupola

44 Cost Comparison (USD-2000) Gabion Rock Mattress with concrete $21,920 Plastic Grid $21,712 Foam Plug $20,890 Gabion Rock Mattress $18,880

45 Thank-You